The present invention relates generally to wireless communication systems and, in particular, to the selection of receiving antennas in wireless communication devices having a plurality of antennas.
Wireless communication systems are well known in the art. In such systems, wireless communication devices typically communicate information (such as voice and data) with each other via one or more wireless channels, e.g., radio frequency (RF) channels. A fundamental performance limitation on such wireless systems, however, arises from the fact that the wireless channels used to convey information are typically subjected to a variety of noise and interference sources. These impairments to the wireless channels degrade overall system performance, for example, by introducing errors into signals demodulated by wireless receivers. While such degradations may be corrected and/or mitigated through a variety of techniques, system designers continue to develop new approaches to combat this problem.
One well-known technique in this regard is sometimes referred to as diversity reception. Such diversity receivers generally comprise a single receiver (meaning, in this case, a device capable of comparing a plurality of received signals) provided with multiple receiving sources in order to receive a given transmitted signal. One type of diversity receiver then attempts to consistently pick the most reliable receiving source, i.e., the signal with the best reception. Thus, if performance of a given receiving source is intermittently impaired, the diversity receiver can optimally identify a better signal source, thereby maintaining or improving receiver performance.
The benefits of diversity receivers are provided, in part, by virtue of spatial diversity between the multiple receiving sources. For example, in wide-area communication systems, diversity receivers often comprise a device referred to as a comparator coupled to a plurality of geographically diverse receivers. The comparator continuously compares the quality of the signals received from each of the diverse receivers and attempts to select the best signal. The resulting signal output by the comparator optimally represents the best possible signal at all times.
A variation of this technique is used in conjunction with portable or mobile communication devices. For mobile applications, spatial diversity through geographically diverse receivers is not possible. Instead, spatial diversity is achieved by equipping the portable or mobile device with a plurality of directional antennas. As known in the art, a directional antenna provides optimal coverage along a specific direction, in contrast to an omindirectional antenna that provides equal coverage in all directions. By diversely aligning the optimal receive paths of the directional antennas, the probability of the wireless device receiving at least one acceptable signal source is increased. The best possible signal is provided at all times by switching between antennas when appropriate. For this reason, this technique is often referred to as antenna switching. Antenna switching as described herein is particularly effective in applications where it is anticipated that reflected signals would be prevalent; for example, indoor applications.
While diversity reception may be employed to improve system performance, a continuing challenge when implementing diversity receivers is how to determine which source should be used at any given moment in order to ensure that received errors are minimized. For example, when the currently used source degrades, the receiver should ideally switch to the best available source. However, it may be that the current source is actually the best available at that moment and switching would only worsen performance. Conversely, even though the current source is acceptable, it may be advantageous to switch to another source that is currently performing even better. Overall, the goal of any diversity reception scheme, including antenna switching, is to minimize the number of errors occurring at a receiver. Ideally, the number of such errors would be zero; however, in reality this goal is unattainable using only diversity reception or, more particularly, antenna switching, because it is entirely possible for a wireless channel to severely degrade during reception of a signal thereby resulting in completely unpredictable errors. The goal, therefore, of any diversity reception technique is actually to reduce the number of errors detected to be the number of undetectable, and therefore unavoidable, errors. Techniques that help system designers achieve this goal would represent an advancement of the art.
The present invention provides a technique for a wireless communication device to select a signal source with which to receive payload data from amongst a plurality of signal sources. Preferably, each signal source is a directional antenna. Each signal source effectively represents a receive channel. The quality of the channel for each receive signal source is determined in part by examining data collected during reception of test data. Substantially simultaneously, payload data is also used to evaluate channel quality for at least one signal source. Thus, a quality metric calculated for any given signal source is preferably the result of measurements made upon test and payload data. In one embodiment of the invention, the parameters used to calculate the quality metric are divided into two sets of parameters, those based on the test data and those based on the payload data. Preferably, the types of parameters in the first set are not identical to the types of parameters as in the second set. Parameters preferably used to determine the quality metric for a signal source comprise jitter, received signal strength, CRC errors and synchronization errors. Selection of a new signal source preferably occurs only when the current payload signal source compares unfavorably with a threshold. By continuously updating quality metrics based on both test and payload data, more reliable assessments can be made of the various signal sources being used. As a result, signal source selection is rendered commensurately more reliable thereby improving received signal quality. These and other advantages will be apparent from the detailed description that follows.